1. Field of the Invention
The present invention relates to a semiconductor device provided with a power semiconductor element, and an electronic circuit using this semiconductor device, and particularly to a semiconductor device having an overcurrent protection function. A semiconductor device according to the present invention is particularly useful as a semiconductor device provided with a power semiconductor element when an overheat protection for the element is required.
2. Description of the Related Art
As a prior art of a semiconductor device provided with a power semiconductor element, a configuration provided with an overheat protection function is disclosed by Japanese Laid-Open Patent Application Publication No. 63-229757. FIG. 6 is a circuit schematic diagram for explaining a semiconductor device related to the prior art disclosed by Japanese Laid-Open Patent Application Publication No. 63-229757.
A power semiconductor element 11 which is comprised of a power MOSFET controls an electric power supplied to a load 12 (resistor RL) from a power supply 13 (voltage Vdd). Then, in order to protect such power semiconductor element 11, an overcurrent protection portion 14 and a temperature protection portion 15 are formed on the same semiconductor substrate, on which this power semiconductor element 11 is formed.
The overcurrent protection portion 14 described above has a transistor 141 which is comprised of a power MOSFET. This transistor 141 comprises a power MOS FET, only a source electrode of which is isolated from the power semiconductor element 11 in a small areas of 1/100 through 1/3000 among an area of the power MOS FET which forms the above power semiconductor element 11, and which has a configuration common to that of the power semiconductor element 11 other than that. In other words, a voltage Vin supplied to a driving signal input terminal is supplied to gates of the power MOS FETs which form the power semiconductor element 11 and the transistor 141 described above in common via a resistor R11 as a potential Va of a point A. In addition, the drain electrodes of the power semiconductor element 11 and the transistor 141 described above are connected to the load 12 in common.
Then, the source electrode of this transistor 141 is grounded via a resistor R12. A drain of a transistor 142 is connected to the point A used as the gate of the transistor 141. This transistor 142 is controlled by a potential Vb of a point B used as a terminal voltage of the above resistor R12. Accordingly, when an overcurrent flows to the power semiconductor element 11, a current proportional to the overcurrent flows to the transistor 141, and when the potential of the point B is thereby increased, the transistor 142 turns on to lower the potential of the point A. As a result, the current which flows to the power semiconductor element 11 is restricted.
The temperature protection portion 15 described above comprises a temperature sensing element 151 to which the above input voltage Vin is supplied via a resistor R13, and this temperature sensing element 151 is grounded via a resistor R14. The temperature sensing element 151 comprises a series circuit of a plurality of polysilicon diodes.
Then, a zener diode 152 is connected in parallel to a series circuit of the temperature sensing element 151 and resistor R14 described above, and a constant voltage is applied to the series circuit of the temperature sensing element 151 and the resistor R14 described above.
In addition, a transistor 153 is arranged in this temperature protection portion 15, this transistor 153 is connected between the above point A and the ground point, and a gate electrode thereof is connected to a node C of the temperature sensing element 151 and the resistor R14 described above.
Herein, description will be made of an operation of the above temperature protection portion 15. When the power semiconductor element 11 generates heat and a temperature of the semiconductor substrate increases, the temperature sensing element 151 will detect this temperature rise. More specifically, when the temperature of the semiconductor substrate increases, a voltage across terminals of the temperature sensing element 151 is decreased, and the potential Vc of the point C is also increased in connection with it. Then, when the temperature of the semiconductor substrate increases not less than a predetermined temperature, and the potential Vc becomes not less than a threshold voltage of the transistor 153, a channel will be formed in this transistor 153 and the transistor 153 will turn on. As a result, the potential Va of the point A will drop. The power semiconductor element 11 will therefore be turned off and this power semiconductor element 11 is protected from thermal destruction.
The above semiconductor device for power is generally driven with system LSIs or microcomputers. In recent years, in accordance with the trend of energy saving, a supply voltage to these system LSIs and microcomputers is becoming further lower from a voltage of 5 V to a voltage of not more than 3 V. In a conventional overheat protection circuit, since the overheat protection is not applied at a constant value in a wide voltage range including the voltage outside 3 V through 5V, both inclusive, for example a range of 1 V through 6 V; it is difficult to obtain sufficient reliability.
The reason is as follows. In order for the overheat protection operation to operate normally according to the voltage across the terminals of the temperature sensing element 151, it is required that a constant voltage is applied to the series circuit of the temperature sensing element 151 and the resistor R4, and the potential Vc of the point C becomes a certain setting value at a predetermined temperature.
However, a zener voltage of the zener diode 152 which determines a voltage of this series circuit is fixed to a constant value while being manufactured. When the zener voltage is set according to an upper limit of the input voltage range, if a voltage significantly lower than the zener voltage is supplied to the driving signal input terminal, a voltage across terminals of the zener diode 152 will drop according to the input voltage.
Therefore, the voltage applied to the series circuit of the temperature sensing element 151 and the resistor R4 will drop, and the potential Vc will also drop lower than the setting value. In such a state, even when the temperature of the semiconductor substrate increases and the voltage across the terminals of the temperature sensing element 151 decreases to a value, at which the overheat protection operates, the potential Vc of the point C has not reached the threshold value of the transistor 153, so that the transistor 153 does not turns on, and the voltage Va of the point A does not drop, thereby the power semiconductor element 11 is not turned off.
In this case, the temperature rises not less than the setting value, and the overheat protection operation is not performed until a rise of the potential Vc due to a decrease of the voltage across the terminals of the temperature sensing element 151 exceeds the threshold of the transistor 153.
For example, supposing that the input voltage is on the order of 5 V and the overheat temperature protection value is 140° C., when the input voltage becomes low on the order of 3 V, due to above reason, the substrate temperature reaches 200° C. at the lowest, and exceeds a guaranteed junction temperature 150° C. of the semiconductor, so that it is considered that reliability thereof is remarkably damaged. On the other hand, when the zener voltage is set according to a lower limit of the input voltage range, the problem described above does not arise.
However, in order to reduce the zener voltage, it is necessary to increase field strength of a PN junction. Specifically, it is necessary to make concentrations of an N type impurity and a P type impurity which are implanted into the substrate high, and for example, when making a zener diode having a zener voltage on the order of 1 V to 2 V, the concentration of each impurity becomes too high, thereby making it extremely difficult to manufacture the same. In particular, under diffusion conditions where the power semiconductor element is manufactured, it is impossible to manufacture such diodes simultaneously.